Script devs, component devs, and analytics devs need the ability to differentiate editor mode from non-editor mode, and adust their scripts & CSS to behave accordingly.
I'd prefer this as a custom attribute of;
<html data-wf-mode="editor" ...
Since that can be easily selected in script, jQuery, and CSS.
This should only appear when in editor mode. In live view mode, that attribute can simply not exist.